In a four-stroke cycle engine, power is recovered from the combustion process in four separate piston movements (strokes) of a single piston, including intake, compression, power, and exhaust strokes. During the intake stroke, the intake valve is opened to introduce air into the combustion chamber. Similarly, during the exhaust stroke, the exhaust valve is opened to allow exhaust to be released from the combustion chamber. During the compression and power strokes, the intake and exhaust valves are maintained closed, thus sealing the combustion chamber and maximizing compression during the compression stroke and efficiently transferring the energy resulting from combustion into piston movement in the power stroke. If the combustion chamber is not sealed during the compression stroke, for example, a cylinder may lose most of its air/fuel mixture prior to ignition leading to what is commonly referred to as misfire.
Misfire is a common problem in internal combustion engines, resulting in poor performance, decreased fuel economy, increased emissions, etc. Misfires may result from poor cylinder compression due to a degraded intake valve, degraded exhaust valve, or degraded piston rings. Accordingly, it is imperative to accurately and easily diagnose whether an underperforming cylinder is a result of intake valve, exhaust valve, or piston ring degradation. Towards this end, an offboard cylinder leakdown test may be performed in which fuel flow is disabled to the indicated cylinder, spark plugs are removed, and shop air is introduced while monitoring for audible air leak. For example, U.S. Pat. No. 7,581,433 teaches an apparatus and method for detecting cylinder leaks in an internal combustion engine including a hose having an end configured for communication with a spark plug hole connecting to a cylinder to be tested and an end for attachment to a source of pressurized gas. However, the inventors herein have recognized potential issues with such a method. For example, the method is such that it must be performed offboard by specialists, and it is additionally intrusive, difficult, and time consuming. An attractive alternative therefore, is a method for an onboard cylinder leakdown test that does not require operator intervention.
In one example, the issues described above may be addressed by a method for a vehicle. During a first condition, including an engine-off condition, a quantity of fuel is injected into a first cylinder of the engine while maintaining an intake valve and an exhaust valve of the first cylinder closed, and indicating degradation of the first cylinder responsive to hydrocarbon migration out of the cylinder. In this way, cylinder degradation may be diagnosed without the requirement for an offboard test conducted by specialists.
As one example, the migration of hydrocarbon out of the cylinder may be indicated responsive to a change in temperature of a fuel vapor canister responsive to a vacuum applied to an engine intake. In this way, the location of the cylinder degradation may be determined, while preventing emissions resulting from hydrocarbon migration out of the cylinder in the vapor canister. It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.